This invention relates to an optical projection aligner for forming circuit patterns on a semiconductor wafer, and more particularly to an aligner having a projection optics with an alignment mark detector which is correctly set at a zero point thereof.
When forming a circuit pattern on a wafer, each pattern of many reticles is projected and developed on the wafer one by one, by using a reduction projection aligner. In each projection of the every reticle patterns on the wafer, a relative alignment between the wafer and a projection optics of the reduction projection aligner is needed.
Generally, the wafer is aligned and fixed on an X-Y stage by using an alignment mark detector set up on the projection optics and detecting an alignment mark thereby on the wafer which is formed in a former process of the projection and the development of the wafer. That is to say, the X-Y stage mounting the wafer is automatically moved so as to correspond the alignment mark on the wafer with the zero point of the alignment mark detector which is the center of the detection area.
By the way, the zero point of the alignment mark detector shifts from a correct position, for example, when a positional drift between the alignment mark detector and the projection optics occurs because of the temperature change. When the zero point shift occurs, an offset error arises between every pattern projecting onto the wafer in spit of the correspondence of the alignment mark of the wafer to the zero point.
Therefore, the X-Y stage is controlled to correct the offset error calculated from a gap between the patterns on the wafer by observing the gap in experimental projections on the wafer using an electron microscope or other measuring equipment before actual projections of real patterns.
Japanese Laid-open Patent No. 59-13324 is cited as an example of such prior art concerning offset error correction.
Furthermore, Japanese Laid-open Patent No. 61-114529 shows a system to detect a latent image of the alignment mark on the wafer which is projected from the reticle. The latent image is formed by only projecting of the alignment mark of the reticle and have a different refractive index from that of other part which is not projected the alignment mark. But the detected latent image is not clear because it is not developed. Therefore, there still remains a problem in a practical use of the above stated detecting system.